Nuclear Chemistry

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NUCLEAR TRANSMUTATION

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Nuclear Bombardment Reactions

• Nucleus can change identity in 2 means:

– Spontaneous decays (radioactive decays)

– Struck by a neutron or by another nucleus

Nuclear transmutations

–nuclear reactions that are induced by striking a nucleus with a neutron or by another nucleus

– changing one element to another by shooting a nuclear particle at its nucleus 3

Nuclear transmutations • Ernest Rutherford (1919) performed the 1st

conversion of one nucleus into another.

• Such rxns made it possible to synthesize hundreds of radioisotopes in the lab.

• Shorthand

notation:

HOHeN 1

1

17

8

4

2

14

7

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Symbols Used in Transmutation Rxn

alpha, or = proton, or = p

neutron, = n electron, or =

positron, or = +

deuterium, = d tritium, = t

He4

2

n1

0e0

1

e01

p1

1 H1

1

5

α4

2

β0

1

β0

1

H2

1 H3

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Learning Check

Write the balanced equation & shorthand notation for the bombardment of boron-10 with an alpha particle emitting a neutron?

ANSWER:

10B + 4He 13N + 1n 10

5B (, n) 137N or 10B (, n) 13N

5 2 0 7

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Practice Exercise

•Write the balanced equation & shorthand notation for the ff when applicable:

1) Bombardment of N-14 with neutron to form C-14

2) 168O + 11H 13

7N + 42He

3) 56Fe + 2H 4He + 54Mn

4) 2713Al(n,) 24

11Na

5) 106Pd(,p)109Ag 7

Practice Exercise Answers!

•Write the balanced equation & shorthand notation for the ff when applicable:

1) 147N + 10n 14

6C + 11H & 147N(n,p)14

6C

2) 168O(p,)13

7N or 16O(p,)13N

3) 5626Fe(d,)54

25Mn or 56Fe(d,)54Mn

4) 2713Al + 10n 24

11Na + 42He

5) 10646Pd + 42He 109

47Ag + 11H

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Transuranium elements

• Elements produced by artificial transmutations of elements with atomic number above 92.

• Named so because occur immediately following uranium in the periodic table

• Created synthetically in particle accelerators.

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Transuranium elements • Np-93 & Pu-94 (1940)

• European scientist (1994)

– Short lived, undergo alpha decay w/n milliseconds

• Element 113 & 115 (2004)

• Element 118 (2006): no names have been chosen 10

Transuranium elements

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Particle accelerators • Device that

accelerates charged particles using strong magnetic & electrostatic fields.

• Popularly called “atom smashers” bear such names as cyclotron and synchrotron.

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Particle accelerators

• Charged particles, such as alpha particles, must be moving very fast to overcome the electrostatic repulsion between them & the target nucleus.

• The higher the nuclear charge on either the projectile or the target, the faster the projectile must be moving to bring about a nuclear reaction.

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NATURAL RADIOACTIVITY

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Radioactivity

– the spontaneous emission by unstable nuclei particles or electromagnetic radiation

• Nuclei outside the belt of stability, as well as nuclei with Z > 83, tend to be unstable.

• Why? (Natural Radioactive Decay)

– The nucleus has many positively charged protons that are repelling each other.

– The forces that hold the nucleus together can’t do its job and the nucleus breaks apart.

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Main types of radiation

• Alpha particle emission

• Beta particle emission

• Gamma radiation emission

• Positron emission (less common)

• Electron capture (less common) 16

Radioactive decay series • A sequence

of nuclear reactions that ultimately result in the formation of a stable isotope.

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FUSION VS FISSION

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Nuclear Binding Energy

• The energy required to break up a nucleus into its component protons and neutrons.

• Represents the conversion of mass to energy that occurs during an exothermic reaction.

• Evolved from studies showing that the masses of nuclei are always less than the sum of masses of nucleons.

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Nuclear Binding Energy

• Example:

• The difference between the mass of an atom & the sum of the masses of its protons, neutrons & electrons is called the mass defect.

• Loss in mass shows up as energy (heat) given off; exothermic (Relativity theory)

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Calculating Nuclear Binding Energy

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• Determining the mass defect

– Add mass of nucleons & subtract from actual

• Conversion of mass defect to energy

– 1 amu = 1.6606 x 10 -27 kg

– E = mc2 (Einstein's Relativity theory)

• Expressing nuclear binding energy as energy per mole of atoms/nucleons

– 6.022 x 1023 nuclei/mol or 1.602 x 19-13 J/MeV

Nuclear Binding Energy of ∆m = 0.0304 u 2.73 x 109 kJ or 28.3 MeV per mol nuclie

Atomic & nuclear scales vs nuclear binding energy

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Nuclear Fission

• Process in which a heavy nucleus (mass number > 200 divides to form smaller nuclei of intermediate mass & one or more neutrons

• Releases large amount of energy because heavy nucleus is less stable than its products

• 1st reaction studied: U-235 bombarded with slow n0

• Energy released per mole = 2.0 x 1013 J (heat of combustion 1 ton coal = 5.0 x 107 J

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Nuclear chain reaction

• Self-sustaining sequence of nuclear fission reactions

• Significant feature in U-235 fission

• To occur: fissionable material must be equal or greater than critical mass (minimum mass required to generate a self-sustaining chain reaction)

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The Atomic Bomb • 1st application of nuclear fission

• How is it made and detonated?

• Critical mass 1kg (formed using conventional explosives)

• Never assembled with critical mass already

• U-235 bomb (tested in Alamogordo, Mexico in July 1945) dropped in Hiroshima, Japan, August 6, 1945

• P-239 bomb exploded over Nagasaki, Japan, August 9, 1945

200,000 death, Japan surrendered 08-14-45 25

Nuclear Fusion

• Combining of small nuclei into larger ones

• Occurs constantly in the sun (made up mostly of H & He)

• Takes place only at very high temp, often called thermonuclear reactions

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The Hydrogen Bomb • Aka thermonuclear bomb

• Not affected by problem on fusion reactor

since objective is all power & no control.

• Contain solid lithium deuteride (LiD)

which are packed tightly.

• Detonation: 2 stages – Fission reaction (provides the required temp) then fusion reaction

• No critical mass, force of explosion is limited only by the quantity of reactants present.

• Cleaner than atomic bombs because only radioisotopes produced (tritium, emitter), unless nonfissionable materials are incorporated such as Co-60 ( emitter)

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Fission vs Fusion

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Fission vs Fusion Fission Fusion

Natural occurrence of the process

Does not normally occur in nature

Occurs in stars, such as the sun

By products of the rxn

Produces many highly radioactive particles

Few radioactive particles, but if a fission “trigger” is used, radioactive particles will result.

Energy ratios Million times > released in chemical rxn

3 to 4 times greater than fission

Nuclear weapon Fission bomb aka atomic bomb or atom bomb

Hydrogen bomb

Conditions requirement

Critical mass of substance & high speed neutrons

High density, high temp environment

Energy requirement

Takes little energy to split two atoms

Extremely high energy to bring two or more protons close enough that nuclear forces overcome their electrostatic repulsion

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NUCLEAR REACTORS

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Nuclear Reactors

• Peaceful but controversial application of nuclear fission

• Device for controlling nuclear reactions so that energy released by the reaction can be converted to a useful form at a constant rate.

• Generation of electricity using heat from a controlled chain reaction in a nuclear reactor.

• Currently, provides about 20% of electricity in US. – Fusion reactors -- breeder reactors -- fusion reactors

• Light water reactors & Heavy water reactors 32

Fission reactors common components

• Fuel

–U-235 (only naturally occurring isotope of U)

–Prepared as pellets of uranium oxide (UO2) & packed in stainless tubes called fuel rods

• Moderator

– Slows down produced neutrons

–Used to absorb some of the energy without absorbing the electrons

– In modern reactors, acts as coolants 33

Fission reactors common components

• Control rods

– Regulate the rate of chain reaction by absorbing neutrons; usually Cd & B

• Shields

– Materials that are good absorbents of radiation

– Concrete & steel (very good at absorbing radiation & equally strong as well)

• Coolants

– Circulated through coils that transfer heat to the water reservoir which produces steam turns turbine runs generator produces electric power

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Major problem of fission reactors • Disposal of high level radioactive wasted

produced in the operation

• Heat energy

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Common Types of Fusion Reactors

• Light water reactors

• Heavy water reactors

Water reactor

Light water reactor

• Fuel: U3O8 (enriched U-235)

• Moderator: light water (H2O)

Heavy water reactor

• Fuel: U-235 not enriched

• Moderator: heavy water (D2O)

• Main advantage: eliminates need for building expensive U enrichment facilities

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Breeder reactor

• Produces more fissionable materials than it uses.

• Uses U fuel which transmutes U-238 into fissionable isotopes

• Problem:

– Economics; breeder reactors are more expensive to build than conventional reactors

– Technical difficulties associated with construction

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Research reactors

• Used for research & training , materials testing, or the production of radioisotopes for medicine & industry.

• Smaller/simpler than power reactors & operate at lower temp

• There are about 240 reactors operating in 56 countries

• Many are on university campuses

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Fusion reactors

• Major concern:

– Temperature necessary to carry out the process

• Some promising rxns:

• Problem:

– thermal pollution

• Advantages:

– Fuels are cheap & almost inexhaustible

– Process produces little radioactive waste

• If turned off, it would shut down completely & instantly, w/o danger of meltdown

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Fusion reactors

• Why isn’t there even one fusion reactor producing energy?

• Basic problem:

– Finding a way to hold the nuclei together long enough & at the appropriate temp, for fusion to occur. (how to contain plasma at temp 100 M°C)

– Solid container that can exist at such temp

• Recent solution:

– Use magnetic confinement (tokamak)

– High-power laser (Nova) 40

Hazards of nuclear energy

• Nuclear power plants are nuclear bombs kept under control at all times

• Environmentalist regard nuclear fission as a highly undesirable method of energy production.

• Many fission products are dangerous radioisotopes with long half-lives

• Risk of accidents

• Problem of radioactive waste disposal 41

Nuclear reactors accidents • 1st: Three Mile Island reactor in Pennsylvania (1979)

– Very little radiation escaped the reactor but the plant remained closed for more than a decade

• Chernobyl in Ukraine (April 26, 1986)

– Surged out of control, resulting in chemical explosion & fire

– People working near the plant died within weeks

– Agriculture & dairy farming were affected by the fallout

– Potential cancer deaths between few thousand to more than 100,000 42

NUCLEAR CHEM PPT- PART 2 ENDS HERE

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